The present invention relates to implantable medical devices for delivering a liquid therapeutic substance to a delivery site within a patient. More particularly, it relates to systems and methods for estimating or indicating a residual volume of therapeutic media contained within a reservoir of an implantable delivery device, and related methods of operating the implantable delivery device.
A variety of implantable medical devices are available for treating patients. For example, implantable therapeutic substance delivery devices are typically used to deliver infusion media or therapeutic substance (such as medication) to a patient at a regulated dosage. The implantable therapeutic substance delivery device (sometimes referred to as a drug pump or medicament pump) is implanted by a clinician into a patient at a location appropriate for the therapy. Typically, an infusion catheter is connected to an outlet of the device, and is implanted/positioned to infuse the therapeutic substance at the desired therapy site so as to treat a condition such as pain, spasticity, cancer, neurodegenerative diseases, trauma, diabetes, and other medical conditions. In addition to the implantable delivery device and the catheter, available intrathecal therapeutic substance delivery systems conventionally include an external programmer that facilitates wireless communication between the delivery device and the clinician. Regardless, the delivery device includes a housing maintaining a reservoir within which the therapeutic substance is contained, along with a pump and/or metering mechanism (and related control circuitry, if necessary) that operates to control flow of the therapeutic substance from the reservoir.
The drug reservoir can assume a variety of configurations, but normally is adapted to impart a positive pressure onto the contained therapeutic substance to better ensure continuous dispensement so long as the reservoir is at least partially filled. For example, one well-accepted design incorporates a bellows assembly as part of the reservoir, and forms a propellant chamber opposite the reservoir. A propellant (e.g., inert gas) within the propellant chamber creates a positive pressure on the bellows assembly that, in turn, positively acts upon the contained infusion media. With this technique, then, as therapeutic substance is withdrawn from the reservoir, the bellows assembly contracts via the external force created by the propellant. One such implantable therapeutic substance delivery device is the SynchroMed® EL implantable drug pump available from Medtronic, Inc., of Minneapolis, Minn. A number of different designs can also be employed that may or may not include a bellows arrangement.
Regardless of exact design, over time, the therapeutic substance in the reservoir becomes depleted and it is necessary to refill the device with a new supply of therapeutic substance. In order to avoid the need for surgery to access and refill the device, it is desirable to have the ability to percutaneously refill the drug reservoir. This is commonly done by providing the delivery device with a fill port assembly that establishes fluid access to the drug reservoir from an exterior of the device. In this regard, a resilient, resealable septum is provided with the fill port assembly, and is accessible by percutaneously inserting a hypodermic needle through the skin and then the septum. Once the septum has been pierced, the hypodermic needle is fluidly connected to the drug reservoir to permit refilling.
While efforts have been made to optimize the refill port design, for example in terms of ease of identification and needle insertion, clinicians may still experience difficulties in determining whether refilling of the reservoir is needed. That is to say, while available devices are capable of ensuring consistent refilling via percutaneous interface, the clinician may not have a complete understanding of how much therapeutic substance is currently contained within the device. While it is possible to track the volume of therapeutic substance dispensed over time (either manually or automatically) and compare the tracked dispensed volume with the reservoir volume when initially filled, a clinician may further desire a confirmation of the actual contained volume. In light of the hermetic, metallic barrier commonly associated with the reservoir along with the importance of preventing contact between the contained therapeutic substance and materials that are otherwise incompatible with the media, no viable device for measuring therapeutic substance reservoir volume has heretofore been available.
In light of the above, a need exists for an implantable therapeutic substance delivery device that is capable of generating information indicative of a current volume of the drug reservoir or a residual volume of the contained therapeutic substance.